Secrets
by KawaiiBlue18
Summary: This is a GB!Underswap!Sans fic in the world of Underswap. Sans is a ten-year-old who isn't allowed to leave home. While his Dad does his job at a local party store, Papyrus is in charge of making sure his little brother stays at home. But while Papyrus was out, an unexpected visit came to their house, following it, Sans finds out something that he had never saw in himself before.
1. Chapter 1: The Lab

**I don't own Underswap or Undertale. Neither do I own the Gaster Blaster Sans AU. Hope you like it anyway! ^^**

 **Chapter 1: The Lab**

(Sans POV)

It was a normal day at home, like I always am. Dad goes off to work while I stay home with my older brother, Papy. I know I'm only 10 years old, but come on, atleast I wanna see what outside is like. Papyrus won't even let me go out as well. I mean sure he keeps me entertained at home while Dad is away, but I just wanna meet new monster, ya know? Maybe someone who doesn't say such horrible puns ALL THE TIME! It gets annoying sometimes. Anyway, I am currently sitting in my room. Papyrus said he would go get us some pizza for lunch and he be right back. I just lying on the floor trying to make magic. Like atleast summon a bone. But no matter how hard I try, it's not working. I sigh. Maybe I'm not trying hard enough. I can do this! The magnificent Sans WILL get as good as Dad and Papy at magic! I get focused, trying with all my might. Until sudden a bone surrounded by a magical blue flame appears in front of me. YES! I must tell Papyrus and Dad when they get home! The only problem is, how do I make it go away? I stand up, that's when I noticed my left eye was a flaming blue while my other eye, didn't have its pupil in it. Will that happen when I always use magic? What color is Paps and Dad's eyes when they do magic? Is it blue like mine, or another color? I had a lot of questions in my mind. I jumped when I heard a knock on the door. Did Papyrus forget to the keys again? I went down stairs to the front door and was about to open it when I hesitated. Dad said never to open the door to strangers. But what if was Papy? I wish we had a window so I could see who is was. There was a hole, but I was too short to reach it. So instead I waited for someone to speak. Papy usually spoke saying he was there. The door knocked again. A voice then came, one that I hadn't heard of before. It was a girl.  
"Gaster? Are you in there? You weren't at work and I know this is your break time. I wanted to speak with you about how everythings going. So if you won't mind me coming in? Gaster? Hello?" The voice said. I didn't know what to do. How did she know Dad? Who is she? She isn't Papy! I tried to sound confident.  
"I don't know who you are or what you want! But, uh, just stay away!" I said trying to sound as courageous as I could be.  
"Ah, you must be one of Gaster's little boys. I will not harm you, please do not be frightened." She said again. She sounded kind, but I kept my guard up.  
"Who are you?"  
"I am Queen Toriel, and you are?"  
"I'm S-Sans, Sans the skeleton. What do you want?"  
"I'm just here to speak with you Father. Say, is he home?"  
"N-no. I'm home alone."  
"I thought you had a bigger brother…? Where is he?"  
"He out getting food. Why would you take so much interest, though? I just met you!"  
"Calm down, little one, as I said before, I will not to harm you."  
"Promise?!"  
"I promise. I just need to see your Dad. Do you know where he is?"  
"N-no, but Papyrus would know! I'll go call him!"  
And with that I left the door and to the kitchen where, with a little trouble cause of my height, I managed o grab the kitchen phone and called Papyrus's number. The phone dialed and dialed, but hit voicemail. Strange. Why won't Papyrus pick up his phone? I shrugged and put the phone down. I walked back to the door.  
"Papyrus won't answer for some reason."  
There was no answer.  
"Hello?" I tried calling to see if she was there.  
No answer again. I shrug. Maybe she left…? But maybe she is hurt? Or something happened or is happening?! Why else would a QUEEN want to see Dad?! I'm so stupid for not seeing this before! I unlocked the door and opened it. A sudden cold rushed in. It was freezing! There was snow everywhere on the floor, but atleast it wasn't snowing. I went back inside to grab one of Paps' spare jackets. I know it was a bit too big on me, but I didn't have any jackets. I stepped outside and closed the door. The snow felt weird under my feet. It made a crunching noise with every step I took. There was nothing dangerous about this. But I could tell why Papyrus always wore his orange jacket. The Queen monster girl left some big footprints on the snow. I decided to follow them. I followed the footprints past many strange buildings, I decided to not let curiosity get the best of me and continue to follow the footprints. The footprints stopped at this weird looking place. It wasn't as inviting as all the other buildings, it looked blank and unhappy and a serious place to be. I wonder why. The footprints led to a back door which was surprisingly unlocked. Hmm. Maybe it wasn't open too long ago. I pushed the door open and made my way inside. The lights inside the building was still on so I could see. It looked like a very long hallway infront of me with doors on both sides. The walls were only white, the floor had only white tiles. What is this place? As I walked down the hall, I saw that all the doors had a different name on each door. As I kept walking, I stopped when I heard talking. One of the voices was the monster girl's voice and there was another monster's voice. It was a girl. She sounded young, but not too young. I followed the voices down the hall and around a corner. The next hall looked similar to the last, but there was a strange room. I found who the voices had belonged to, entering the strange bigger room. The Queen girl, was some sort of goat monster wearing a purple cloak with gold shoulder plates. The other monster was a teenage fish-like monster with red hair in a ponytail. She was wearing a strange white coat. I didn't bother to listen on what they were saying. Maybe Dad was here? I snuck behind them, trying not to be seen as I followed them inside. The room was weird looking. Instead of white, the walls were all grey and there was 5 computers going down a row, next to a lot of buttons. The Queen was pointing out the buttons, probably explaining them. Infront of the computers and buttons, there was a glass wall. I got on a chair to have a better look. There was other monsters behind the glass, in another room, it looked like they were making some kind of machine. Those monsters also wore the same strange coat. I got down from the chair after seeing the Queen and monster girl had started walking again. I followed them. They opened a door into the strange room behind the glass. Many of the monsters working on the machine stopped and either waved or bowed at the Queen. I stepped into the room, looking around the room confused. I froze when I heard one of the monsters in the white coats call to the Queen, "Hey your magisty, I think a little someone was following you."

(Papyrus POV)

I was returning home with some pizza in hand. I couldn't wait to see that cute smile of Sans' when I brought home one of his favorite foods. When I reached the door, I saw that the door was unlocked. I opened the door which a little panic. "Sans?! I'm home! I brought pizza!" I called. But no answer. I put the pizza down on the table next to my forgotten phone. Too lazy to ever remind myself to bring it with me. I then walked upstairs to Sans' room to find, that he wasn't in his room. Though there was a random bone on the floor. I bent over to pick it. I could easily feel that it had been made with Sans' magic. I smiled. He finally was able to create something with his magic! But where is he? I started to get more panicked as I checked the whole house for him, but I couldn't find Sans. Oh no! That's why the door was unlocked! Sans is gone! DAD'S GONNA KILL ME!

 **Hope you liked this chapter! ^^**


	2. Chapter 2: Muffets

**I don't own Underswap or Undertale. Neither do I own the Gaster Blaster Sans AU. Hope you like it anyway! ^^**

 **Chapter 2: Muffets**

(Sans POV)

Everyone starred at me and I could only freeze in fear. The Queen had turn around along with the fish girl to look at me. The Queen softened her expression as she looked at me.  
"Oh, it's just you, Sans. What are you doing here? Hey, Albert, (lol random monster) could you escort our little friend back to his home please? He is Gaster's little boy," She said to a tall cat-like monster who had white fur. He nodded and grabbed my hand. He then walked me out of the room, I looked back reaching out for the Queen as he pulled me to walk away from them. I needed to know what was wrong and what was going on and what was this place? And where was Dad?! The cat monster named Albert brought me back out of the building, he let go of me once we were outside.  
"Ok kid, now go back home. This is important stuff for adults only. I'm sure that Dad of yours is waiting for you back home," He said. I looked at him confused. He had said 'that Dad of yours' with disgust. He looked down at me sternly before heading back inside the building, leaving me out in the cold. The only problem was, it was starting to snow and I don't remember how I got here. Maybe the nearby buildings can help me. So I started walking toward where I think the building were. The snow was getting heavier, but I could still see some lights up ahead. I started running, if I wanted to get out of this cold quicker, I'm gonna have to move faster. I was almost there, I started making out the shape of the building. It was wooden with lights on inside. I could see some other monsters inside through its open doors. I was almost there, until I tripped and plummeted into the snow, face first. I got up shaking of the snow on my face. Burr! It's freezing! I walked up to the wooden building, shivering, and I walk to the door and stopped to look inside. It looked so festive. There was a bunch of monsters all sitting at tables, drinking, laughing, and talking. No one seemed to notice me. I walked inside, past the tables and monsters and up to a taller, longer table ahead of me. I tried climbing up one of the tall seats. With a few tries I managed to get on top of the seat. I got up on a sitting position and looked across the table. The monsters sitting at the long table with me, still didn't seem to notice me. Maybe someone here could help me. All of the sudden, when I wasn't paying attention, a voice said to me,  
"Hello Papy- wait, you aren't Papyrus."  
I turned forward to see a spider lady monster standing on the other side of the long table. She was wearing some kind of short sleeve suit. I could only stare at her. How did she know Papyrus?  
"I'd recognize that jacket of Papyrus' anywhere, but his jacket ain't ever too big for him. You must be his little brother Sans I've heard about."  
"How do you know Papy? And how do you know my name?" I tried asking.  
"Your brother Papyrus, comes by here every so often for a drink or food. He's told me a lot about you. You're lucky to have such a brother who loves you very much. Oh and by the way, my name is Muffet. I run this restaurant. So, welcome to Muffets, which is the name of my restaurant by the way."  
"Um thanks…"  
"Say, are you hungry? I could grab you a kids meal burger. It'll be on the house," she said, winking with her two right eyes. I had just noticed how hungry I was. Though I don't really have a stomach. Papy said that food just disappears in our bodies. But wait, Paps said he would bring me pizza. So I decided to decline her offer.  
"N-no thanks. Papyrus said he would bring me pizza when he got home. Though I'm not sure h-how to get h-home."  
"Are you lost?"  
I nodded.  
"Well we can always call Papyrus-"  
"I did! He didn't answer his phone though."  
"Well Papyrus is pretty forgetful, gotta admit that. Maybe he forgot his phone at home."  
"Yeah maybe…."  
"Well I'll tell you what, you can hang around here until Papyrus shows up, cause I know he will come here anyway. And while you wait, how about you have a lollipop?" She grabbed a lollipop from her pant pocket and handed it to me. I took it and starred at it. I've seen Papyrus eat these before. Though he never gives me one, saying I will get too hyper if I do. I unwrapped and put it in my mouth anyway.

(Papyrus POV)

I grabbed my stuff and tried searching all over. It was hard to see because of the heavy snowfall. I tried finding footprints, but the snow would have already coved them up. I called all over for Sans, asking some monsters on my way. Sans must be freezing! He doesn't really have a jacket of his own since he's never been outside before. Oh Sans. I'm sorry. I took a break and headed over to Muffets, the only place I could go at the moment.


	3. Chapter 3: The Blue Glow

**I don't own Underswap or Undertale. Neither do I own the Gaster Blaster Sans AU. Hope you like it anyway! ^^**

 **Chapter 3: The Blue Glow**

(Papyrus POV)

I walked to Muffets, in the cold and in grief that I hadn't watched Sans closer. Maybe I can rethink my thoughts on where he might be over some honey and a cigarette. Yeah, that'd be nice. I got to the door and walked inside. The place was full like always and went up and sat at the bar. I didn't bother to look around. It's not like he is here anyway. Muffet spotted me and came over to me.  
"I'll have the usual," I said.  
"Hey, I've been waiting for you! How could you leave your brother like that?!"  
I looked up at her confused, "What'd do you mean?"  
"Sans, does that ring a bell in your lazy brain?"  
"Sans is here?!"  
"Yeah, he was wearing your jacket, he's in the back. Come on." She opened the door on the side of the bar. I got up and followed her to the back door, where she led me into the kitchen and food closet. It was a mess. I was surprised at first since I knew she was very tighty and liked things organized. I looked at Muffet. Her expression was surprised as well, she had probably not made that mess. Huh this place now kinda reminds me of my bedroom when Sans or Dad doesn't try to clean it, now that I think about it.  
"I just cleaned this yesterday!" Muffet scowled at no one while still looking at the mess.  
"Where is Sans?" I asked trying to get the point why she dragged me back here. "Cause he's not here."  
"He was! I took him back here not to long ago. Did he cause this mess?"  
"I dunno. He is usually really organized, but hyper when he eats sweets-…you didn't give him sweets did you?"  
"It was only a small lollipop!"  
"That explains it. This is why we don't give him sugar. The last time he ate something sweet, he was still a baby bones and just learning how to walk, I think I was like 12 or 13, I don't remember. Dad came home from work that night and bought us nice cream. I was fine when I ate it, but Sans was all over the place, jumping and running around, even if he tripped and fell at some points. His eyes even glowed to show how much energy he felt. It was around midnight when he finally burned out all the energy and we were able to put him to sleep. That's when Dad decided not to give him sweets anymore. That was back before, when Sans could glow both of his eyes."  
"What do you mean both of his eyes?"  
"Well Sans' right eye is broken. He can still see out of it, but he can't glow his eyes with that pupil anymore."  
"What happened?" Muffet sounded more concerned now. She can be really attached with children so I dunno if that's just how older girls respond when they see little kids, or it's just Muffet being Muffet.  
So I continued anyway. "Well when Sans was about 1-" *CRASH!* We both jumped at the loud noise. We rushed that way, behind the stoves, there he was. There was flour everywhere on the floor, and in the middle of it, sat Sans. A bucket that had the label 'Flour' on it was on top of Sans' head, practically covering his face and head. He was wearing his usual clothes, with my spare, now-flour-covered, orange, jacket. A muffled giggle could, be heard from under the bucket. I walked over and pulled the bucket off his face. Sans smiled up at me, happily. He was also glowing his left eye, blue. That part saddened me since, it was a while since I'd seen Sans' eye glow his blue colored magic, mine is an orange glow by the way.  
Sans giggled, "Papyrus!" And he gave me a big hug. I smiled and hugged him back.  
"P-Papyrus….," Muffet whispered, in a some what scared/in horror tone. What got her so scared all the sudden? I let go of the hug to look at her.  
"What?" I asked her casually. Her face expression was in horror as she starred at…Sans?  
"L-Look…," she pointed at Sans with one of her hands. I turned to look at Sans. Then I saw what she meant.

(Sans POV)

I felt so happy an hyper after I ate that lollipop. I felt as energy rushed through me. I ran around the place after Muffet told be to stay back here, I may have made a bit of mess running around and crashing into stuff. That's when I heard Paps' voice. I got excited and bumped into a shelf. Next thing I know, a bucket of flour falls on me with a crash. I hear footsteps rushing my way and someone removes the bucket off my head, revealed to be Papyrus. I gave him a hug and then Muffet starred at me in shock for some reason. That's when I noticed I was getting some kind of headache. The back of my head and my teeth hurt like fire. Papyrus pulled me out of the hug and starred at me with shock and concern. My body started heating up with pain. Everything hurt, mostly my spine, legs, hands, and head.  
"P-Papyrus…wh-why does e-everything hurt…?" I tried asking my big brother through the pain. I looked down at my hands, with look of shock, I saw that I had claws for fingernails. My hands were also flaming blue. I started to cry and panic. What was going on?! Papyrus only starred at me in shock and horror. What's wrong with me?! The pain continued for what felt like half and hour and Papyrus did nothing but but his hands on my shoulders. During that time, I felt and heard bones cracking and snapping. What was happening to me?! The pain finally stopped after some time and I fell and layed down on the floor, my hands covering my face which was pouring with tears. My body was sore and shaking like crazy. I was so scared. I felt Papyrus pick me up and pull me into a hug. I clung my arms around him, not wanting to let go. I couldn't move. I didn't want to move. Why…? Is this why Dad said not to go outside, because he knew this would happen? I should have listened then! I'm so stupid! I started crying in Papyrus's jacket (the one he was wearing). Papyrus rubbed my back and trying to soothe me I guess.  
As he passed Muffet, he said, "I gotta get him to Dad…" I fell asleep in Papyrus's arms after that. The pain was too much to keep my eyes open.

 **Poor Blueberry, but I hope u guys like the story so far!**


	4. Chapter 4: Gaster

**I don't own Underswap or Undertale. Neither do I own the Gaster Blaster Sans AU. Hope you like it anyway! ^^**

 **Chapter 4: Gaster**

(Muffet POV)

I starred in shock and horror when I saw poor little Sansy cry in pain. His body had started changing, and it was not a pleasant change. Horns had started growing out the back of his head, his bone started cracking above his nose, he grew sharp, pointy teeth too, and I think other things might have changed, but they were covered by Papyrus' spare jacket that Sans was wearing. Oh, poor Sans. After some time, the blue glow surrounding Sans, disappeared and Papyrus picked him up and Sans wrapped him arms around Papyrus's neck. Papyrus then started rubbing Sans' back and Sans I think, went to sleep, his left eye still flaming blue even though his eyes were closed. Papyrus got to his feet and carried Sans to the back door.

"I gotta get him to Dad….," Papyrus said as he past me. I opened the back door for him and then closed it.

"I hope he feels better soon!" I called as Papyrus walked out into the falling snow keeping Sans close to him.

(Papyrus POV)

This has got to be literally the weirdest day of my life. What was happening to Sans?! This hadn't happened the last time he ate sugar! Well I gotta get him to Dad. Dad works at a Party store not to far from here. It's one of the only joyful places here in Snowdin other than Muffets. Dad is an excitable person, but also busy since his store his pretty busy during the day. Monsters can still be able to have joyful parties despite the loneliness of Snowdin and the rest of the Underground. It was way to spot his store, cause of all the lights. I walked into the store, closing the door behind me.

Dad, who was behind the pay counter, not looking at us, said, "Welcome to the Party Store!"

"Dad," I called out.

He turned around to look at me.

"Oh hey Papyrus, what brings you here and why do you have Sans with you? Isn't he supposed to stay inside the house?"

"Yeah, but something's wrong with him. He started changing into something. I don't know what. I'm scared for him."

"Let me see him," Dad demanded. I removed Sans arms from my neck and gently handed him over to Dad.

(Gaster POV)

It had been an unexpected visit from BOTH of my sons. First of all, why was Sans out of the house? Second of all, why is Sans' eye glowing? I can see blue light coming from him even though he isn't facing me.

"Let me see him," I demanded. I needed to see what was wrong. Papyrus detached Sans from him and gave him over to me gently. I took him in my arms. Even though Sans was 10 years old, his lack of growth spurt made him seem very small in Papyrus' jacket. I took off the hoodie from his head. I gasped at what I saw. No no no no no no no no no no! This can't be happening!

"What's wrong with him?" Papyrus asked after noticing my panicked reaction.

"I knew something like this would happen…," I said shedding some tears.

"You knew this would happen and you didn't tell us?"

I didn't respond.

"Please Dad, tell me. I'm as worried as you are about Sans, and I can't help if I don't know anything."

"I s-suppose. Well might as well start from the beginning. Remember that night when I was fight training as a Royal Guard? You were too young for me to just leave you alone at home."

"I remember that. You were fighting against that dinosaur girl. Alphys I think her name was."

"Yes, Alphys. She was young and still needed training before she became a royal guard yet. She was strong but clumsy. I'll never forget. Well while I had summoned some gaster blaster attacks-"

"Sans got lose out of my arms. I tried to follow him, but Sans was to quick going down the stairs and ended up tripping and tumbling down the stairs through the door to the battle field. I think he wanted you."

"I shot a gaster blaster blast at Alphys, but she used on of her axes to deflect it, sending the blast right at Sans. I signaled a stop to our fight after noticing what the blast had hit. I ran over to Sans in panic. Gladly, he was still alive, but he was crying and his left eye glowed blue. That's when I noticed that the blast had broken his right eye from glowing. I check his HP still amazed and thankful he was still alive. His HP was dangerously low yet, I could feel a power in him that I had never seen in him before. I picked him up, trying to heal him. By that time, you had already rushed in next to Sans, trying to comfort him since you did not know how to heal yet. I managed to get his HP back to full health, but the I could not heal the broken eye. I quit being a royal guard trainer after that, fearing you or Sans might get hurt if I left you unattended. When we got home that night. I tried searching the library for anything on what would happen if a skeleton survived a gaster blaster blast since I did not know everything about a skeleton's magic. And no, I did not create the gaster blaster, your grandparents just named me after it, cause I know you asked me a while back why my name was on one of the main skeleton attacks. Anyways, I found some books on skeleton magic and brought them home. After looking and searching a bunch of times, I found something. It said that after the blast of a gaster blaster upon a skeleton, they are most likely to either die or if they survived, they would take physical changes of a gaster blaster on their body if they allowed a lot of their energy and magic to flow through them. With this fear, I decided to keep Sans away from about learning his magic at all. I kept him locked up at home, cause if he went outside, someone might tell him about his magical ability. Also Sans can also trigger him magic and energy by sweets which will make him jumpy and can trigger what has happened now, which is the second reason why you don't give him anything sweet. Though one day, I left one of the books about magic lying on the couch, Sans had read it learning he, you, and I could do magic, but even with the new power flowing in him that he did not know of, a skeleton's first summon with magic is always the hardest, even when summoning a bone. After that it's much easier to use magic. Which reminds me, how did this happen? Why was Sans using magic? What for? Start explaining Papyrus."

 **Lol, looks like Paps is in trouble. Anyway hope you like this chapter! Reviews would be appreciated.**


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